Crude oils, their straight-run and cracked fractions and other petroleum products contain sulphur in varying amounts, depending upon the source of the crude oil and any subsequent treatment that it may have undergone. Besides elemental sulphur, numerous sulphur compounds have been identified in crude oil including hydrogen sulphide (H.sub.2 S), C.sub.1 to C.sub.5 primary alkyl mercaptans, C.sub.3 to C.sub.8 secondary alkyl mercaptans, C.sub.4 to C.sub.6 tertiary alkyl mercaptans, cyclic mercaptans (such as cyclopentane thiol, cyclohexane thiol and cis-2-methylcyclopentane thiol), open chain sulphides of the formula R--S--R' where R and R' represent C.sub.1 to C.sub.4 alkyl groups, mono-, bi- and tri-cyclic sulphides, thiophene, alkyl substituted thiophenes, condensed thiophenes (such as benzo(b)thiophene, isothionaphthene, dibenzothiophene, and benzo(b)naphtho(2,1-d)thiophene), thienothiophenes, alkyl cycloalkyl sulphides, alkyl aryl sulphides, 1-thiaindans, aromatic thiols (such as thiophenol), and cyclic thiols such as cyclohexane thiol.
Generally speaking, low API gravity crude oils usually contain more sulphur than high API gravity crude oils, although there are some exceptions. Moreover the distribution of sulphur compounds in the different fractions of petroleum varies mainly with the boiling range of the fractions. Thus the lighter fractions such as naphtha contain fewer sulphur compounds, whilst the content of sulphur compounds also increases as the boiling point or API density or molecular weight of the fraction increases. Most of the sulphur compounds that have been positively identified as components of crude oil boil below about 200.degree. C. Many other sulphur compounds of high molecular weight and high boiling point remain unidentified in crude oil.
For a variety of reasons it is necessary to treat crude oil and petroleum fractions derived therefrom to remove the sulphur components present therein. Otherwise subsequent processing may be hindered, for example because the sulphur components may adversely affect the performance of a catalyst. If the hydrocarbon fraction is intended for fuel use, then burning of the fuel will result in any sulphur components present therein being converted to sulphur oxides which are environmentally damaging.
For these reasons it is necessary to remove as far as possible the sulphur content from hydrocarbon fractions derived from crude oil, such as gasoline fractions, diesel fuel, gas oils and the like. Typically such sulphur removal is carried out by a process known generally as hydrodesulphurisation. In such a process the hydrocarbon fraction is admixed with hydrogen and passed over a hydrodesulphurisation catalyst under appropriate temperature and pressure conditions. In such a process the aim is to rupture the carbon-sulphur bonds present in the feedstock and to saturate with hydrogen the resulting free valencies or olefinic double bonds formed in such a cleavage step. In this process the aim is to convert as much as possible of the organic sulphur content to hydrocarbons and to H.sub.2 S. Typical equations for major types of sulphur compounds to be hydrodesulphurised are shown below: ##STR1##
Generally the cyclic sulphur-containing compounds are harder to hydrogenate than the open chain compounds and, within the class of cyclic sulphur-containing compounds, the greater the number of rings that are present the greater is the difficulty in cleaving the carbon-sulphur bonds.
Besides the presence of sulphur oxides in the combustion gases from hydrocarbon fuels, other environmentally undesirable components of such combustion gases typically include aromatic hydrocarbons, which may be present because of incomplete combustion, and carbonaceous particulate matter often containing polycyclic aromatic hydrocarbons, metal compounds, oxygenated organic materials, and other potentially toxic materials.
Because of present concerns about pollution, increasingly stringent limits are being placed by various national legislations around the world upon the levels of permitted impurities in hydrocarbon fuels, such as diesel fuel. In particular the United States Environmental Protection Agency has recently proposed rules which would limit the sulphur content to 0.05 wt % and the aromatics content to 20 volume % in highway diesel fuels (see, for example, the article "Higher Diesel Quality Would Constrict Refining" by George H. Unzelman, Oil and Gas Journal, Jun. 19, 1987, pages 55 to 59). Such rules require refiners to face additional diesel treating requirements and increased investment and operating costs. Additional reductions in the permitted levels of sulphur content and aromatics content at some future date cannot be ruled out.
When a hydrocarbon feedstock is treated with hydrogen in the presence of a suitable catalyst with the aim of effecting hydrodesulphurisation, other reactions may also occur. Hence hydrotreating is often used as a more general term to embrace not only the hydrodesulphurisation reactions but also the other reactions that occur, including hydrocracking, hydrogenation and other hydrogenolysis reactions. The term "hydrotreating" is further explained in an article "Here is a nomenclature-system proposed for hydroprocessing", The Oil and Gas Journal, Oct. 7, 1968, pages 174 to 175.
There are four main hydrogenolysis reactions, of which hydrodesulphurisation (HDS) is probably the most important, followed by hydrodenitrogenation (HDN), hydrodeoxygenation (HDO), and hydrodemetallation (HDM). Amongst catalysts which have been proposed for such hydrotreating reactions are molybdenum disulphide, tungsten sulphide, sulphided nickel-molybdate catalysts (NiMoS.sub.x), and cobalt-molybdenum alumina sulphide (Co-Mo/alumina).
Although the prior art regards the simultaneous occurrence of some hydrogenation reactions, such as hydrogenation of olefins and aromatic hydrocarbons, as not being advantageous in a hydrodesulphurisation process because the aromatic content of the product was within the required specification and because the use of valuable hydrogen for unnecessary hydrogenation reactions was deemed disadvantageous, there is a growing shortage of light crude oil. Thus the present and future trend towards the use of middle distillates and heavier petroleum fractions, coupled with increasingly stringent specifications, means that aromatic hydrogenation will be an increasingly necessary component of refinery operations. Hence, under current conditions and increasingly for the future, it will be desirable to combine hydrodesulphurisation and aromatic hydrogenation.
In contrast, except when processing high molecular weight residues, extensive hydrocracking reactions are to be avoided in most refinery hydrotreating operations as far as possible because they are highly exothermic and can cause thermal damage to catalysts and reaction vessels, as well as leading to the deposition of carbonaceous materials causing loss of catalyst activity. Thus an operator of a hydrodesulphurisation plant has reported in an article "Refiners seek improved hydrogen production", Oil & Gas Journal, Jul. 20, 1987, pages 48 and 49, that reactors in service have overheated severely, one to the point of rupture, due to unwanted hydrocracking reactions occurring.
The danger of such hydrocracking reactions occurring can be minimised by ensuring that the catalyst remains adequately sulphided.
A number of papers have appeared in the literature relating to hydrodesulphurisation technology, including:
(a) "Kinetics of Thiophene Hydrogenolysis on a Cobalt Molybdate Catalyst", by Charles N. Satterfield et al, AIChE Journal, Vol. 14, No. 1 (January 1968), pages 159 to 164; PA0 (b) "Hydrogenation of Aromatic Hydrocarbons Catalysed by Sulfided CoO-MoO.sub.3 /gamma-Al.sub.2 O.sub.3. Reactivities and Reaction Networks" by Ajit V. Sapre et al, Ind. Eng. Chem. Process Des. Dev, Vol. 20, No. 1, 1981, pages 68 to 73; PA0 (c) "Hydrogenation of Biphenyl Catalyzed by Sulfided CoO-MoO.sub.3 /gamma-Al.sub.2 O.sub.3. The Reaction Kinetics", by Ajit V. Sapre et al, Ind. Eng. Chem. Process Des Dev, Vol. 21, No. 1, 1982, pages 86 to 94; PA0 (d) "Hydrogenolysis and Hydrogenation of Dibenzothiophene Catalyzed by Sulfided CoO-MoO.sub.3 /gamma-Al.sub.2 O.sub.3 : The Reaction Kinetics" by D. H. Broderick et al, AIChE Journal, Vol. 27, No. 4, July 1981, pages 663 to 672; and PA0 (e) "Hydrogenation of Aromatic Compounds Catalyzed by Sulfided CoO-MoO.sub.3 /gamma-Al.sub.2 O.sub.3 " by D. H. Broderick et al, Journal of Catalysis, Vol. 73, 1982, pages 45 to 49. PA0 "It is a fundamental kinetic fact that at pressures for normal middle distillate desulfurizers (500 to 800 psig) it is difficult to obtain appreciable aromatic saturation. Thus, if the feedstock is far above the 20% aromatics level, there is not much you can do with typical hydrotreaters, with any catalysts that we have knowledge of, to significantly reduce aromatics. PA0 You are then left with the unpalatable alternatives of higher pressure units, aromatic extraction, and all the other alternatives."
A review of the reactivity of hydrogen in sulphide catalysts, such as those used as hydrotreating catalysts, appears on pages 584 to 607 of the book "Hydrogen Effects of Catalysis" by Richard B. Moyes, published by Marcel Dekker, Inc. (1988).
A review of industrially practised hydrotreating processes is published each year in the Journal "Hydrocarbon Processing", normally in the September issue. For example reference may be made to "Hydrocarbon Processing", September 1984, page 70 et seq and to "Hydrocarbon Processing", September 1988, pages 61 to 91.
An outline of three prior art hydrotreating processes appears in "Hydrocarbon Processing 1988 Refining Handbook" on pages 78 and 79 of "Hydrocarbon Processing", September 1988. In the "Chevron RDS/VRDS Hydrotreating Process" a mixture of fresh liquid hydrocarbon feedstock, make-up hydrogen and recycled hydrogen is fed to a reactor in a "once-through" operation. As illustrated the reactor has three beds and inter-bed cooling is provided by injection of further amounts of recycle hydrogen. The recycle hydrogen is passed through an H.sub.2 S scrubber. In the "HYVAHL Process" a once-through operation for the liquid feed is also used. Again, amine scrubbing is used to remove H.sub.2 S from the recycle hydrogen. The Unionfining Process also utilises a once-through basis for the liquid feed. Co-current hydrogen and liquid flow is envisaged. Unreacted hydrogen is recycled.
In all three processes gas recycle is used to cool the catalyst bed and so minimise the risk of thermal runaways occurring as a result of significant amounts of hydrocracking taking place. Use of gas recycle means that inert gases tend to accumulate in the circulating gas which in turn means that, in order to maintain the desired hydrogen partial pressure, the overall operating pressure must be raised to accommodate the circulating inert gases and that the size and cost of the gas recycle compressor must be increased and increased operating costs must be tolerated.
Use of a trickle technique is described in an article "New Shell Hydrodesulphurisation Process Shows These Features", Petroleum Refiner, Vol. 32, No. 5 (May 1953), page 137 et seq. FIG. 1 of this article illustrates a reactor with four catalyst beds with introduction of a mixture of hot gas and gas oil at the inlet end of the first bed and use of cold shots of gas oil between subsequent beds.
In these hydrodesulphurisation processes the conditions at the inlet end of the catalyst bed are critically important because this is where the risk of hydrocracking is greatest, especially if the level of sulphurisation of the catalyst should drop. This can occur, for example, if a low sulphur feedstock is fed to the plant or if a feedstock is used in which the sulphurous impurities are predominantly polycyclic compounds.
Hydrorefining of a naphtha feedstock is described in U.S. Pat. No. 4,243,519. This appears to involve a substantially wholly vapour phase process.
Multiple stage hydrodesulphurisation of residuum with movement of catalyst between stages in the opposite direction to movement of gas and liquid is described in U.S. Pat. No. 3,809,644.
U.S. Pat. No. 3,847,799 describes conversion of black oil to low-sulphur fuel oil in two reactors. Make-up hydrogen is supplied to the second reactor but in admixture with hydrogen exiting the first reactor that has been purified by removal of hydrogen sulphide therefrom. Hence hydrogen is recovered from the first reactor and recycled to the second reactor in admixture with inert gases which will accordingly tend to accumulate in the gas recycle loop. Any condensate obtained from the first reactor is admixed with product from the second reactor.
In a hydrodesulphurisation plant with a gas recycle regime some of the H.sub.2 S produced, normally a minor part thereof, will remain in the liquid phase after product separation whilst the remainder, normally a major part thereof, of the H.sub.2 S will remain in the gas phase. Even in plants in which interbed cooling with "cold shots" of recycle gas is practised the H.sub.2 S released remains in the gas/liquid mixture as this passes through the catalyst bed. Hence the H.sub.2 S partial pressure is usually highest at the exit end of the catalyst bed or of the final bed, if more than one bed is used. As the catalyst activity for hydrodesulphurisation is decreased by raising the H.sub.2 S partial pressure, the catalyst activity is lowest at the exit end from the bed which is where the highest activity is really needed if the least tractable polycyclic organic sulphurous compounds are to undergo hydrodesulphurisation.
The catalysts used for hydrodesulphurisation are usually also capable of effecting hydrogenation of aromatic compounds, provided that the sulphur level is low. The conditions required for carrying out hydrogenation of aromatic compounds are generally similar to those required for hydrodesulphurisation. However, as the reaction is an equilibrium that is not favoured by use of high temperatures, the conditions required for hydrodesulphurisation of cyclic and polycyclic organic sulphur compounds in a conventional plant do not favour hydrogenation of aromatic compounds. Moreover as the design of conventional hydrodesulphurisation plants results in high partial pressures of H.sub.2 S at the downstream end of the plant the catalyst activity is correspondingly reduced and the conditions do not lead to significant reduction in the aromatic content of the feedstock being treated. Hence in an article entitled "Panel gives hydrotreating guides", Hydrocarbon Processing, March 1989, pages 113 to 116, it is stated at page 114:
Removal of H.sub.2 S from a hydrodesulphurisation plant with a gas recycle system is normally effected by scrubbing the recycle gas with an amine. As the scrubber section has to be sufficiently large to cope with the highest levels of sulphurous impurities likely to be present in the feedstocks to be treated, the scrubber equipment has to be designed with an appropriate capacity, even though the plant will often be operated with low sulphur feedstocks. The capital cost of such scrubber equipment is significant.
It would be desirable to provide a more efficient process for effecting hydrodesulphurisation of liquid hydrocarbon feedstocks, in particular one in which the danger of hydrocracking reactions occurring is substantially obviated. It would further be desirable to provide a hydrodesulphurisation process in which the activity of the catalyst is controlled throughout the reactor in such a way that improved levels of hydrodesulphurisation can be achieved at a given operating pressure than can be achieved in a conventional process. It would also be desirable to provide a hydrodesulphurisation process which permits operation in such a way as to achieve a simultaneous significant reduction in the aromatics content of the feedstock being treated, particularly those feedstocks in which the aromatics content exceeds about 20%.